The primary components of ink jet printheads are a semiconductor chip, a nozzle plate, and a flexible TAB circuit attached to the chip. The semiconductor chip is preferably made of silicon and contains various passivation layers, conductive metal layers, resistive layers, insulative layers and protective layers deposited on a device side thereof. For thermal ink jet printers, individual heater resistors are defined in the resistive layers and each heater resistor corresponds to a nozzle hole in the nozzle plate for heating and ejecting ink toward a print media.
Typically, the chip is mounted to a printhead body within a chip window on a flexible TAB circuit. The TAB circuit attaches to a print head body and provides electrical contact pads for connecting to corresponding contacts in the ink jet printer. The TAB circuit includes many closely-spaced electrically-conductive traces that connect the print head chip to the contact pads. Typically, metal leads span the chip window to connect the traces to connection points on the chip. The metal leads and connection points on the chip are susceptible to mechanical damage during the manufacture of the printhead and during normal use of the printhead. The metal leads are also susceptible to corrosion damage from exposure to ink once the printhead has been installed on a printer.
For example, with regard to corrosive damage, ink supply channels within the chip receive ink from an ink reservoir in the print head cartridge. Through capillary action, the ink flows into the channels and is provided to ink ejection elements on the chip. The ink-ejection elements are selectively activated to cause ejection of ink droplets toward a print medium. Due to the close proximity of the chip to the source of the ink, and due to the low viscosity of the ink, the ink tends to flow around the edges of the chip and come in contact with the leads and the traces. Many formulations of ink are somewhat conductive and corrosive. When a space between two leads of a TAB circuit is filled with such ink, and an electrical potential exists between the leads, an electrical current may flow through the ink from one lead to the other. This current flow causes electrochemical corrosion of the source lead, that is, the lead that is the source of the current flow. The corrosion narrows the lead over time, and eventually corrodes the lead completely through, rendering the print head chip partially or completely inoperable.
Conventionally, an encapsulant is used in an effort to protect the leads from mechanical and corrosion damage. However, improvement is desired in the construction of printheads, particularly in regard to protection of the leads from mechanical and corrosive damage.